This invention relates to the drilling of a relief well and more particularly, to locating the relative trajectory of said relief well with respect to the blowout well.
Typically, wells are drilled into the earth's crust to desired subsurface oil-and/or gas-bearing formations. A drilling mud is pumped downwardly through a rotating drill string within the well, through the drill bit at the bottom of the drill string, and thence upwardly to the surface of the well through the annulus surrounding the drill string. A "blowout" may occur when the well penetrates a high pressure gas-producing formation due to a number of circumstances. Gas from the high pressure formation may enter the well and mix with the drilling mud so that its density is reduced by gas occlusion, thus reducing the hydrostatic head on the well to a value less than that of the formation pressure. A blowout may also occur during removal of the drill string from the well. Displacement of the drilling mud by the drill string may result in a decrease in the liquid level within the well with, again, a decrease in the hydrostatic head at the level of the high pressure formation.
When a blowout occurs, a number of remedial procedures are available to kill the blowout and bring the well under control. One technique involves the drilling of a relief well into a subsurface formation near the blowout well. Communication between the relief well and blowout well is established and fluids are then pumped down the relief well and into the blowout well in an attempt to impose a sufficient hydrostatic head to block the flow of gas from the formation into the well. Communication between the wells may be established through the high pressure sand which caused the blowout or through a separate permeable zone penetrated by both the blowout and relief wells. U.S. Pat. No. 4,224,989 Blount describes the killing of a blowout well in this manner.
A problem arises in drilling the relief well into close proximity with the blowout well in a formation in which communication between the relief well and the blowout well can be established. Techniques of directional drilling are known and described, for example, in U.S. Pat. No. 4,220,214--Benoit. Instruments are available for obtaining precise surveys of the trajectories of the well. U.S. Pat. No. 3,229,375--Crake, et al is an example. It is usually quite difficult to drill the relief well to close on the blowout well based solely on survey measurements from the wells.
Magnetic ranging equipment such as that described in U.S. Pat. No. 4,072,200 has been successfully used to track the trajectory of the blowout well with respect to the relief well. However, this technique is useful only when the blowout well has casing or drill pipe in the hole. Frequently, magnetic ranging data cannot be obtained because the interval of interest has no iron in the hole.
Acoustic techniques are sometimes used to track the trajectory of one well with respect to another. U.S. Pat. No. 3,722,605--Isham shows an example of an acoustic logging instrument used for this purpose. These tools are not suitable for use where the wells are widely spaced or in subsurface formations, which are not conducive to accoustic logging.
It is an object of the present invention to accurately establish the relative trajectory of a relief well with respect to a blowout well.